Thermally Activated Delayed Fluorescence and Phosphorescence Quenching in Iminophosphonamide Copper and Zinc Complexes

Abstract The synthesis of copper and zinc complexes of four variably substituted iminophosphonamide ligands is presented. While the copper complexes form ligand‐bridged dimers, the zinc compounds are monomeric. Due to different steric demand of the ligand the arrangement of the ligands within the dimeric complexes varies. Similar to the structurally related iminophosphonamide complexes of alkali metals and calcium, the steady‐state and time‐resolved photoluminescence (PL) of four of the seven compounds studied here as solids in a temperature range of 5–295 K can be described within the scheme of thermally activated delayed fluorescence (TADF). Accordingly, they exhibit bright blue‐green phosphorescence at low temperatures (<100 K), which turns into delayed fluorescence by increasing the temperature. However, unusually, the fluorescence is practically absent in two copper complexes which otherwise still conform to the TADF scheme. In these cases, the excited singlet states decay essentially non‐radiatively and their thermal population from the corresponding low‐lying triplet states efficiently quenches PL (phosphorescence). Three other copper and zinc complexes only exhibit prompt fluorescence, evidencing a wide variation of photophysical properties in this class of compounds. The excited states of the copper complex with especially pronounced phosphorescence quenching were also investigated by low‐temperature time‐resolved infrared spectroscopy and quantum chemical calculations.


Experimental section 1.1 General Procedures
All manipulations of air-sensitive materials were performed under the rigorous exclusion of oxygen and moisture in flame-dried Schlenk-type glassware either on a dual manifold Schlenk line, interfaced to a high vacuum (10 -3 torr) line, or in an argon-filled MBraun glove box. Hydrocarbon solvents (toluene, n-pentane) were dried by using an MBraun solvent purification system (SPS-800), degassed and stored under vacuum.
n-hexane and thf were predried over CaCl2 before decantation, distillation from potassium benzophenoneketyl and storage over 4 Å molecular sieves. MeOH was predried over CaCl2 before decantation, distillation from magnesium turnings and storage over 3 Å molecular sieves. Dichloromethane (dcm) was distilled from phosphorus pentoxide and stored over 4 Å molecular sieves. Copper(I)chloride was obtained from abcr, Potassium bis(trimethylsilyl)amide 95% and Diphenylzinc were obtained from Sigma Aldrich, the 1.2 molar dimethylzinc solution in toluene was obtained from Acros Organics. All purchased chemicals were used without further purification.

Nuclear Magnetic Resonance (NMR) Spectroscopy
NMR spectra were recorded on a Bruker Avance II 300 MHz or Avance III 400 MHz. Chemical shifts were measured relative to the characteristic solvent resonances as internal standards [7.16 ppm ( 1 H) and 128.06 ppm ( 13 C)]. They are expressed in parts per million (ppm) and reported relative to tetramethylsilane. 85% phosphoric acid was used as external reference for 31 P-and 31 P{ 1 H}-NMR. 1 H-NMR spectra are reported as follows: chemical shift ( in ppm), multiplicity (br for broad singlet, s for singlet, d for doublet, q for quartet, m for multiplet), coupling constant(s) (Hz), number of protons (concluded from the integrals), specific assignment. 13 C{ 1 H} NMR spectra are reported in terms of the chemical shift and specific assignment. If necessary, NMR assignments were made using a combination of 1D and 2D techniques [ 1 H-1 H COSY, 1 H-13 C HMQC, and 1 H-13 C HMBC].
All compounds were thoroughly dried before examination of the NMR spectra to remove any disturbing solvent signals if possible.

Elemental Analysis (EA)
Elemental analyses were carried out with a Vario Micro Cube (Elementar Analysensysteme GmbH).

Infrared (IR) Spectroscopy
IR spectra were obtained on a Bruker Tensor 37 FTIR spectrometer equipped with a room temperature DLaTGS detector, a diamond ATR (attenuated total reflection) unit, and a nitrogen flushed chamber. In terms of their intensity, the signals were classified into the categories vs = very strong, s = strong, m = medium, w = weak and vw = very weak.

Electron Ionization (EI) Mass Spectrometry
EI mass spectra were recorded at 70 eV on a Finnigan MAT 8200 instrument.

Single Crystal X-ray Diffraction
A suitable crystal was covered in mineral oil (Aldrich) and mounted on a glass fiber. The crystal was transferred directly to the cold stream of a STOE IPDS 2 or a STOE StadiVari diffractometer. All structures were solved by using the programs SHELXS/T [4] and Olex2 [5] . The remaining non-hydrogen atoms were located from successive difference Fourier map calculations. The refinements were carried out by using full-matrix least-squares techniques on F2 by using the program SHELXL. [4a] In each case, the locations of the largest peaks in the final difference Fourier map calculations, as well as the magnitude of the residual electron densities, were of no chemical significance.
Crystallographic data for the structures reported in this paper have been deposited at the Cambridge Crystallographic Data Centre as a supplementary publication no. 2075954-2075960. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB21EZ, UK (fax: (+(44)1223-336-033; email: deposit@ccdc.cam.ac.uk).

Photoluminescence (PL) Measurements
PL measurements were performed with a Horiba Jobin Yvon Fluorolog-322 spectrometer equipped with a closed-cycle optical cryostat operating within a temperature range of ca. 5-300 K. A Hamamatsu R9910 photomultiplier was used as detector for the emission spectral range of about 300-830 nm. The solid samples (crystalline powders) were measured as dispersions in a thin layer of viscous polyfluoroester oil (ABCR) placed between two 1 mm quartz (Spectrosil® 2000) plates. The latter were mounted on the cold finger of the cryostat. All emission spectra were corrected for the wavelength-dependent response of the spectrometer and detector (in relative photon flux units). Emission decay traces were recorded by connecting a photomultiplier to a 500 MHz LeCroy LT322 oscilloscope (via a 50, 500, 2.500 or 10.000-Ohm load depending on the decay time scale) and using a nitrogen laser (~2 nsec, ~5 μJ per pulse) for pulsed photoexcitation. For compound 7 a quadrupled Nd:YAG laser at 266 nm excitation wavelength was applied. PL efficiencies of solid complexes at ambient temperature were determined using an integrating sphere out of optical PTFE, which was installed into the sample chamber of the spectrometer, according to the method of de Mello et al. [6] The uncertainty of this measurement was estimated to be ±10%.
For TADF, the energy separation E between S1 and T1 states can be estimated from the temperature dependence of PL decay time, (T), within the simple model of the thermally equilibrated S1 and T1 states, according to equation S1: [7] ( ) = 3 + exp(− ∆ ) where kT is the thermal energy, (S1) and (T1) are the intrinsic fluorescence and phosphorescence lifetimes, and statistical factor 3 takes into account three T1 substates. Note that this model does not account for temperature dependences of both radiative and nonradiative relaxation processes. The assumption of the thermally equilibrated S1 and T1 states implies that the forward and reversed intersystem crossing which couples S1 and T1 is much faster than any other (non)radiative relaxation process involving these states. intensity outside the measured region should be observed). FTIR ground state spectra were recorded systematically to check if there is no sample degradation.

Theoretical Methods
The molecular structure of compound 3 from X-ray crystallography was used as input structure and geometry optimizations were performed with the Berny algorithm of Gaussian 09 [8] by using energies and gradients computed by Turbomole 7.4. [9][10] All calculations were performed with the DFT functional B3LYP with dispersion correction (no three-body interaction) (D3(BJ)) [11] as implemented in Turbomole using the resolution of identity (RI) approximation and the def2-TZVP basis set. The lowest triplet state was optimized as a ground state by unrestricted DFT (UDFT). Harmonic frequency calculations were performed for the optimized minimum structures. The vibrational frequencies are scaled by a factor of 0.975 to minimize the differences between the experimental and calculated frequencies. A Gaussian convolution with a full-width at half-maximum of 8 cm −1 was applied to the calculated vibrational transitions.
Additionally, the energetically lowest excited singlet and triplet states were optimized by time-dependent DFT with the same functional and basis set as in the ground state. All the indicated relative energies are corrected by the zero-point vibrational energy.
Single crystals suitable for X-ray structure analysis were obtained in form of colourless needles after recrystallization from hot n-hexane.  CuCl (38.0 mg, 0.38 mmol, 1.00 eq.), (R)-HDippPEPIA (183.0 mg, 0.38 mmol, 1.00 eq.) and KBTSA (76.0 mg, 0.38 mmol, 1.00 eq.) were dissolved in toluene (30 mL) and were stirred at room temperature for 16 h. After filtration the filtrate was concentrated under reduced pressure, which yielded a crystalline solid. The resulting colorless solid was washed with n-pentane (5 mL). Single crystals suitable for X-ray analysis were obtained from hot toluene. The solvent was decanted, and the product was washed with cold n-pentane (5 mL  Following a similar procedure as described above for 2 the reaction of CuCl (74.0 mg, 0.74 mmol, 1.00 eq.), H-DippPIA (400.0 mg, 0.74 mmol, 1.00 eq.) and KBTSA (149.0 mg, 0.74 mmol, 1.00 eq.) afforded single crystals from toluene suitable for X-ray analysis. The solvent was decanted, and the product was washed with cold npentane (5 mL) before drying under vacuum and isolation as colorless solid.

S10 [Zn{(R)-PE Dipp PIA}2] (6)
ZnPh2 (220.0 mg, 1.0 mmol, 1.00 eq.) and (R)-HPE Dipp PIA (481.0 mg, 2.0 mmol, 2.00 eq.) was dissolved in toluene (30mL) and stirred afforded for 16 h. The solvent was removed under reduced pressure and product was washed with n-Pentane. The single crystals suitable for X-ray analysis was obtained from n-Pentane. The solvent was decanted, and the product was washed with cold n-pentane (5 mL) before drying under vacuum and isolation as colorless solid.  [Zn{DippPIA}Ph] (7) ZnPh2 (147.0 mg, 0.28 mmol, 1.00 eq.) and H-DippPIA (150.0 mg, 0.28 mmol, 1.00 eq.) were dissolved in toluene (30mL) and stirred for 16 h. The solvent was removed under reduced pressure and the product was washed with n-pentane. Single crystals suitable for X-ray analysis were obtained from n-pentane. The solvent was decanted, and the product was washed with cold n-pentane (5 mL) before drying under vacuum and isolation as colorless solid.                                             Fig. S6.8). The red curve corresponds to the fit according to eq. (1). The fit yielded the energy separation ΔE between S1 and T1 states of about 500 cm -1 . Below T ≈75 K the emission is predominantly phosphorescence with the lifetime of 44 μsec. Increasing the temperature activates the TADF mechanism, resulting in the effective PL lifetime of ca. 0.51 µsec at room temperature. Figure S7.11. PL decay time of solid complex 2 vs. temperature. The red curve corresponds to the fit according to the model of equilibrated S1 and T1 states (eq. S1). The fit yielded the energy separation ΔE between S1 and T1 states of about 250 cm -1 . Below T ≈75 K the emission is phosphorescence with the lifetime of 47 μsec. Increasing the temperature activates the TADF mechanism, resulting in the effective PL lifetime of ca. 0.17 µsec at 150 K. Figure S7.12. PL decay time of solid complex 3 vs. temperature. As indicated, the decay time varies from 280 sec at 5 K to 0.15 sec at 150 K. The red curve represents the fit according to the model of equilibrated S1 and T1 states (eq. S1). The fit yields the energy separation ΔE between S1 and T1 states of 964  90 cm -1 and the intrinsic S1 lifetime (S1) of about 32 psec. Such extremely short S1 lifetime is another indicator that the assumption of the thermally equilibrated S1, T1 states fails for 3. The 'kinetic' model of the thermally activated quenching of the phosphorescent T1 state -via fast internal conversion of the S1 state populated from T1 -appears in this case more suitable (eq. 1, see text). The blue dashed curve corresponds to the fit according to eq. 1. It yields the energy separation ΔE(S1-T1) = 953  90 cm -1 (as indicated in the Figure) and the pre exponential factor (A in eq. 1) of 8.710 9 sec -1 . The both fit curves are practically identical due to the very large variation of the PL decay time in 3 (over three orders of magnitude within a narrow temperature interval of ~70-120 K). This variation is described by the exponential factor exp(-E/kT) in the denominator of both eq. 1 and S1 -yielding practically the same values of ΔE.